In a centrifugal compressor used in a compressor portion of a vehicular or marine turbo charger or the like, kinetic energy is imparted to a fluid via the rotation of an impeller, the fluid is discharged outwardly in a radial direction, and an increase in pressure by a centrifugal force is thereby achieved. The centrifugal compressor is required to have a high pressure ratio and high efficiency in a wide operation range, and hence an impeller 05 provided with a splitter blade (short blade) 03 between adjacent full blades 01 as shown in FIGS. 23 and 24 is often used, and the blade shape of the impeller 05 is modified in various manners.
In the impeller 05 having the splitter blade 03, the full blade 01 and the splitter blade 03 are alternately disposed on the surface of a hub 07, and the common splitter blade 03 has a shape obtained by simply removing the upstream portion of the full blade 01.
In the case of the common splitter blade 03, as shown in FIG. 25, an entrance end edge (LE2) of the splitter blade 03 is positioned at a predetermined distance on the downstream side of an entrance end edge (LE1) of the full blade 01, exit end edges (TE) of both of the full blade 01 and the splitter blade 03 are provided so as to match each other, and a blade angle θ of the entrance end edge of the splitter blade 03 (shown as an angle formed between the direction of the entrance end edge and an axial direction G of the impeller 05) is set to match a direction F of flow of a fluid flowing in a flow path between the full blades 01.
However, as shown in FIG. 25, when the entrance end edge of the splitter blade 03 is designed to have a shape obtained by simply removing the upstream portion of the full blade 01 at the center between the full blades 01 in a circumferential direction, a difference represented by A1<A2 occurs between a throat area A1 on the side of a positive-pressure surface Sa of the full blade 01 and a throat area A2 on the side of a negative-pressure surface Sb thereof which are formed on both sides of the splitter blade 03, and hence there has been a problem that flow rates of individual flow paths become nonuniform, the fluid cannot be equally distributed, blade loads become unequal, loss of the flow path is increased, and an improvement in impeller efficiency is prevented. Note that the throat area denotes a cross-sectional area at a position where the distance from the entrance end edge of the splitter blade as shown in FIG. 25 to the positive-pressure surface or the negative-pressure surface of the full blade 01 is shortest.
To cope with the problem, the technology disclosed in Patent Document 1 (Japanese Patent Application Laid-open No. H10-213094) is known. In Patent Document 1, as shown in FIG. 26, the blade angle θ of the entrance end edge of a splitter blade 09 is increased to θ+Δθ (θ is increased by Δθ relative to the direction F of flow of the fluid), i.e., the entrance end edge thereof is displaced toward the negative-pressure surface Sb of the full blade 01, the throat areas in passages on both sides of the splitter blade 09 are thereby made equal to each other (A1=A2).
In addition, as the technology in which the entrance end portion of the splitter blade is inclined toward the negative-pressure surface of the full blade, Patent Document 2 (Japanese Patent Publication No. 3876195) is also known.
However, as in Patent Document 1 (FIG. 26), when the blade angle θ of the entrance end edge of the splitter blade 09 is increased to θ+Δθ, there has been apprehension that a separated flow from the front edge portion of the splitter blade 09 having the increased inclination or the negative-pressure surface Sb of the full blade 01 occurs. In addition, even when the throat areas in the passages on both of the positive-pressure surface side and the negative-pressure surface side of the splitter blade 09 are made equal to each other (A1=A2), there has been a problem that a difference in flow velocity between the passages does not allow the equalization of the flow rate.
That is, since the flow velocities on both sides of the splitter blade 09, i.e., the flow velocity on the positive-pressure surface side of the full blade 01 and that on the negative-pressure surface side thereof are different, the fluid having entered between the full blades 01 has a distribution in which a fast flow is concentrated mainly on the negative-pressure surface side so that, even when the flow path cross-sectional areas in the passages on both sides of the splitter blade 09 are made equal to each other geometrically, there has been a problem that the flow rate is increased on the negative-pressure surface side to be higher than that on the positive-pressure surface side due to the higher flow velocity on the negative-pressure surface side than that on the positive-pressure surface side, the flow rates in the individual flow paths become nonuniform, the fluid cannot be equally distributed, the blade loads become unequal, the loss of the flow path is increased, and an improvement in impeller efficiency is prevented.
Further, in the technology disclosed in Patent Document 3 (Japanese Patent Application Laid-open No. 2002-332992), as shown in FIG. 27, without changing the blade angle θ of the entrance end edge of a splitter blade 011, the front edge is displaced toward the negative-pressure surface of the full blade 01, and A1>A2 is thereby satisfied. With this arrangement, the equalization of the flow rate in passages on both sides of the splitter blade 011 is achieved.
Patent Document 1: Japanese Patent Application Laid-open No. H10-213094
Patent Document 2: Japanese Patent Publication No. 3876195
Patent Document 3: Japanese Patent Application Laid-open No. 2002-332992
However, in any of the technologies disclosed in Patent Documents 1 to 3, the improvement of the blade shape is made by focusing on the flow rate distribution of the flow paths obtained by splitting by the splitter blade on the assumption that the flow between the blades flows along the full blade. In the case of an open impeller having a blade-end clearance, an influence by a blade-end leakage flow which flows in or out of a passage from the blade-end clearance is seen, its flow field is complicated, and a further improvement to cope with the complicated internal flow has been required.
The complicated internal flow has been evaluated by numerical analysis, and it has been revealed that a leakage vortex occurring from the tip portion of the entrance end edge of the full blade (the tip portion in a direction of height of the blade from a hub surface (shroud side)) reaches close to the tip portion of the entrance end edge of the splitter blade (the tip portion in the direction of height of the blade from the hub surface (shroud side)) (see a vortex flow of a blade-end leakage flow W in FIG. 22).
The leakage vortex dose not flow along the full blade and the leakage vortex is a place where a low-energy fluid is accumulated, and hence, when the leakage vortex interferes with the entrance end edge of the splitter blade, loss generation resulting from the occurrence of separation and a vortex structure is increased.
That is, in the conventional impeller structure, a countermeasure against the interference between the leakage vortex from the tip of the entrance end edge of the full blade and the entrance end edge of the splitter blade is not taken, and hence adequate performance has not been obtained.